In vacuum tubes for high power transmitters, it is often desirable to be able to switch the tube on to full power rapidly. Tubes, however, employ electron emitting cathodes which must be heated before they emit. The problem of switching the tube to full power rapidly then hinges on the ability to heat the cathode rapidly.
State-of-the-art fast warm-up cathodes are formed by sintering a heater to a low-mass cathode. The heater is made with cataphoretically coated tungsten insulated with an Al.sub.2 O.sub.3 ceramic material. The sintering is usually done at 1300.degree. C. using a mixture of 95% tungsten with 5% nickel. For small tubes, this approach is workable since the cathodes are small, usually 0.25 inches diameter or less. Sometimes a mix of molybdenum and ruthenium is used instead of W-Ni. The sintering temperature is then approximately 1600.degree. C.
For larger cathodes, 0.5 inches diameter or higher, this approach becomes less workable. The problems become unacceptable for cathodes of greater than 1 inch diameter.
The problems can be illustrated by considering a requirement for a 1 megawatt klystrode tube with a 10 second warm-up time. The cathode would have to be about 2.5 inches diameter. The heater would have to heat the cathode inself, the heater wire, the insulating coating, the sintering material and the cathode support. To heat such a large cathode to operating temperature would require 15,000 joules. This amount of energy requires high currents and high voltages. The voltage across the Al.sub.2 O.sub.3 would exceed the breakdown voltage of the material. In addition, currents of the order of 100 amperes have to be delivered to the active heater area. The connections then would have to be substantial conductors which would carry away heat and increase the current requirement further to compensate for the heat loss.
An additional problem in the prior art heaters is the great differences between the coefficients of expansion of the tungsten and the Al.sub.2 O.sub.3. The different rates of expansion cause stress during heating which results in fatigue and failure.
To reduce the requirements for energy in a fast warm-up cathode, as illustrated in the example above, a bombarder heater is often used. An example is shown in U.S. Pat. No. 4,675,573. The bombarder is a heated emitting structure placed behind the cathode. There is a significant electric field between the bombarder and the cathode. Electrons emitted from the bombarder are accelerated into the back of the cathode to heat the cathode.
A quick-heating cathode for an electron tube is described in U.S. Pat. No. 3,299,317 to J. W. Kendall, Jr. In this cathode a wire braid is connected in series with the cathode cylinder. The braid has a high electrical resistance when hot and a low electrical resistance when cold, thus permitting large amounts of current to initially surge through the braid to heat the cathode directly at turn-on. After the initial high current surge, the braid becomes hot and its electrical resistance becomes high. When the braid is hot, less current passes through if for direct heating of the cathode; however, at this time the braid also heats the cathode indirectly due to its high electrical resistance.
A further fast-heating cathode for an electron tube is disclosed in U.S. Pat. No. 2,996,643 to F. C. Johnstone et al. In this cathode arrangement a first voltage is initially applied across a filament spaced from the back surface of the cathode, causing the filament to emit thermionic electrons. A second voltage applied between the filament and the cathode accelerates the emitted electrons to the back surface of the cathode. These electrons bombard the back surface of the cathode to produce rapid heating of the cathode. After the cathode reaches electron emission temperature, the voltage between the cathode and filament is removed, and thermal radiation from the filament maintains the cathode at its operating temperature.
U.S. Pat. No. 4,675,573 issued Jun. 23, 1987 to Miram et al, and assigned in common with the present patent, discloses a fast warm-up cathode arrangement in which the cathode is directly heated with a burst of current through the cathode and then heated from behind by a heater coil.